Semi-closed circuit underwater breathing apparatus have been in use for over twenty years. These apparatus recycle a diver's exhaled breath through a carbon dioxide scrubber and add oxygen back into the circuit to replace the oxygen metabolized by the diver. The partial pressure of the oxygen in the circuit must be controlled based on depth; if the partial pressure is too low the diver can suffer from hypoxia which can lead to unconsciousness and death. On the other hand, in dives exceeding 10 meters in depth, the diver can suffer from oxygen toxicity if the oxygen partial pressure is too high which can lead to central nervous system damage, convulsions and death. Existing mechanical breathing apparatus that are used for depths exceeding 10 meters use two cylinders to provide the breathing gas; one is oxygen while the other is a diluent gas which is blended with the oxygen in a regulator, commonly referred to as a ratio regulator, to provide a safe gas mixture based on depth. A cross section of an existing ratio regulator 10 is shown in FIG. 1.
Regulator 10 provides a fixed flow of oxygen 11 into mixing chamber 12 regardless of depth via an absolute pressure regulator 14 and fixed oxygen metering orifice 16 and a variable flow of diluent gas 13 (e.g., N2 or He gas) that is a function of depth via diluent valve 18, diluent orifice 20 and diluent regulator 22 which is referenced to ambient pressure. Regulator 10 uses two elastomeric diaphragms 24, 26 to trap air in a chamber 28 to provide a reference pressure of 1 atmosphere absolute (1 ATA) and a third diaphragm 30 to form diluent chamber 32 which provides diluent pressure loading when the depth exceeds approximately 6 meters. The force balance resulting from gas pressures and diluent offset spring 34 acting on valve assembly 36 provides a gas mix 15 that is 100% oxygen at depths less than 6 meters and increasingly diluted with nitrogen or helium enriched gas as the depth increases.
Current ratio regulators, such as regulator 10, have several drawbacks. First, in order to meet the operating pressure requirements, the diaphragms are fabric reinforced material which have inconsistent properties. Small differences in thickness and stiffness are sufficient to alter the force balance enough to allow the oxygen concentration to move outside a specified range. Preloading past the normal use length for a specified length of time is often needed to stabilize the performance enough to insure that the equipment can be calibrated within specification.
Secondly, due to the relatively large diaphragm area, helium within diluent chamber 32 can diffuse through the diaphragm and into the 1 ATA reference chamber 28 thereby causing a shift in the partial pressure of the breathing mix. In order to prevent this, it is necessary to use third diaphragm 30 to form a chamber 38 which is then vented to ambient via hole 40. Hole 40 is exposed to salt water and debris which fill the regulator chamber and may result in plugging of hole 40, as well as requiring more effort to clean regulator 10 during maintenance after each use.
Lastly, the assembly of the three diaphragms 24, 26, 30 and five compression plates 42, 44, 46, 48, 50 is difficult and presents a large number of leak paths which can negatively affect reliability and performance.
Thus, there is a need for a ratio regulator that is not susceptible to salt water contamination, thus resulting in faster and easier assembly, testing and maintenance. The present invention addresses these, as well as other, needs.